1. Field of the Invention
The present invention relates to a photomask blank and a photomask for use in the fabrication of semiconductor integrated circuits and high-density integrated circuits.
2. Prior Art
Photolithographic processes involving the use of photomasks are employed in the fabrication of high-density semiconductor integrated circuits such as LSI and VLSI chips, color filters for charge-coupled devices and liquid-crystal displays, and magnetic heads.
Photomasks used for microfabrication are produced from a photomask blank composed of a transparent substrate such as quartz glass or aluminosilicate glass and a light-shielding film, typically in the form of a chromium film, deposited thereon by a sputtering or vacuum evaporation technique. The photomask is created by forming a specific pattern in the light-shielding film of the photomask blank.
The chromium-based light-shielding film has a high light reflectance. To prevent light which reflects back from the semiconductor substrate being exposed and passes through the projection lens from being reflected again by the photomask and returning to the semiconductor substrate, an antireflective coating is generally formed on the surface side, or both the surface and back sides, of the light-shielding film.
The photomask blanks from which photomasks are formed generally have two and three-layer structures. The two-layer structure photomask blanks include a light-shielding film, typically in the form of a chromium film, deposited on a synthetic quartz substrate by sputtering or vacuum evaporation, and an antireflective film deposited on the surface of the chromium film for preventing the exposure light reflected by the silicon wafer from being reflected again. The three-layer structure photomask blanks are arrived at by forming an additional antireflective film on the substrate side.
While a variety of photomask blanks are known in the art, JP-B 62-37385 discloses a photomask blank comprising a transparent substrate on which have been successively formed a chromium carbide-nitride film containing chromium carbide and chromium nitride as an underside antireflective coating, a chromium film as a light-shielding film, and a chromium oxide-nitride film containing chromium oxide and chromium nitride as a surface antireflective coating. The prior art also teaches the use of CrON (JP-B 61-46821 and JP-B 62-27387) and CrN (JP-B 62-27386 and JP-B 62-27387) as the antireflective coating. Also, a single-layer film of chromium nitride is disclosed in JP-B 4-1339.
As the level of integration and miniaturization of high-density semiconductor integrated circuits has continued to rise, the process of inspecting defects in photomask blanks and circuit patterns is now carried out at a high detection sensitivity. Accordingly, the light-shielding film and antireflective film are required to have a high uniformity of quality.
An important requirement of photomasks is that the substrate be flat to assure accurate transfer of the pattern. Yet, no matter how flat a substrate is used, forming a light-shielding chromium base film on the substrate tends to exacerbate the surface state, especially in the two-layer structure, because the light-shielding film is characterized by growth of large grains on the substrate surface. Since the current defect inspection and circuit pattern inspection technologies generally utilize light reflected or transmitted by the surface, the advanced inspection at a high detection sensitivity can detect a degraded surface state of film or rough surface as defects, which suggests the undesired prevention of high sensitivity inspection.
An object of the invention is to provide a high-performance photomask blank and photomask having a sufficient uniformity of film quality to accept high-sensitivity detection for the defect inspection and circuit pattern inspection and capable of accurately forming a desired micropattern without distortion.
The invention is directed to a photomask blank or photomask comprising a transparent substrate, at least one layer of light-shielding film and at least one layer of antireflective film both on the substrate. It has been found that when a seed layer is formed between the transparent substrate and the light-shielding film or the antireflective film, the light-shielding film or the antireflective film is improved in surface flatness whereby a photomask blank or photomask having uniformity of film quality is obtained.
The invention provides a photomask blank comprising a transparent substrate by which exposure light is transmitted, at least one layer of light-shielding film and at least one layer of antireflective film both on the substrate, and a seed layer disposed between the transparent substrate and the light-shielding film or the antireflective film. The seed layer is formed of a chromium material containing at least one of oxygen, nitrogen and carbon. Typically the seed layer has a thickness of 0.5 to 10 nm. Preferably, the light-shielding film or the antireflective film is formed of a chromium material containing at least one of oxygen, nitrogen and carbon. The photomask blank desirably has a surface roughness (RMS) of up to 0.9 nm.
Also contemplated herein is a photomask fabricated by lithographically patterning the photomask blank.
According to the invention, the seed layer on the transparent substrate presents growth nuclei for the light-shielding film or the antireflective film so that the film grows on the seed layer while creating fine grains. Then the interposition of the seed layer eventually endows the photomask blank or photomask with a reduced surface roughness.